1. Field of the Invention
The present invention relates to a process for biochemically purifying contaminated water containing BOD (Biochemical Oxygen Demand) sources and nitrogen sources. More particularly, the present invention relates to a process for purifying contaminated water by microorganisms which decompose BOD sources and nitrogen sources in the contaminated water.
2. Description of the Prior Art
According to conventional biochemical treatment of water containing BOD sources and nitrogen sources such as ammonia nitrogen, nitrous nitrogen and nitric nitrogen, BOD sources are first decomposed by BOD oxidizing bacteria in an aeration tank, the liquid is treated in a nitrifying tank where large quantities of nitrifying bacteria are present to oxidize ammonia nitrogen to nitrous acid or nitric acid, and the treated liquid is anaerobically treated in a denitrifying tank to reduce nitrous acid or nitric acid to molecular nitrogen by denitrifying bacteria in the presence of a hydrogen donor such as methanol or acetic acid.
In this method, it is necessary to arrange an aerobic BOD-treating tank, a nitrifying tank and an anaerobic denitrifying tank, and the treatment apparatus is complicated. Furthermore, if the pH value of the liquid in the nitrifying tank is reduced below 6.5 by conversion of ammonia nitrogen to nitrous or nitric nitrogen, the activity of the nitrifying bacteria in the nitrifying tank is drastically reduced. Accordingly, in order to maintain a high efficiency in the treatment, it is preferred that the pH value of the liquid in the nitrifying tank be maintained at 7 to 8. For this purpose, it is necessary to add an alkaline agent to adjust the pH value in the tank. Furthermore, the nitrifying bacteria undergo concentration inhibition by nitrous acid or nitric acid, and therefore, it is not permissible to increase the concentration of nitrous acid or nitric acid in the tank. Accordingly, the concentrations of ammonia nitrogen, nitrous nitrogen and nitric nitrogen to be introduced into the nitrifying tank should be controlled at low levels. For this reason, contaminated water having a high concentration of ammonia nitrogen, such as urine or waste water from a swinery, should be diluted to reduce the concentration of nitrous acid or nitric acid to be formed and a large quantity of water should be used for dilution.
In the denitrifying tank, nitrous acid or nitric acid is reduced to molecular nitrogen by the action of denitrifying bacteria. To realize this conversion, an expensive hydrogen donor such as methanol or acetic acid must be added in an amount about 3 times the theoretical amount.
As is seen from the foregoing description, the conventional method is defective in various ways. The treatment equipment is complicated increasing the manufacturing cost of the equipment, large quantities of an alkaline agent and an expensive hudrogen donor have to be used, and furthermore, for contaminated water containing high concentrations of ammonia nitrogen, nitrous nitrogen and nitric nitrogen, a large quantity of water must be used for dilution.
A method and apparatus for purifying water containing organic pollutants using a membrane of hollow fiber has been proposed, in which oxygen is supplied through the hollow fibers of the membrane to grow aerobic microorganisms on the membrane and polluted water is purified by the microorganisms (U.S. Pat. No. 4,181,604). In this previous proposal, there are specifically employed aerobic microorganisms, and BOD and COD (Chemical Oxygen Demand) in polluted water are removed by BOD-oxidizing bacteria. However, this previous disclosure does not include a method of oxidizing nitrogen compounds such as ammonia, formed by the action of the BOD-oxidizing bacteria, by nitrifying bacteria and reducing formed nitrous acid or nitric acid to harmless nitrogen products by denitrifying bacteria.